[Exim] message_size_limit exceptions

I need to put an upper limit to the message size for all except a few
users on my email system.
Preferably I'd like to use an option, something like
message_size_limit_except_senders like there is for
recipients_reject_except_senders, but I couldn't find anything.

One thought I had was to construct a test like this:

${if and
{>{$message_size}{10M}}{!lookup{$sender_address}lsearch{can_send_large_mail}}{message_size_limit
= 10M}{message_size_limit = 0}}}

Would this test be ok, or is there an easier way?

Thanks,

Rolf Habing
Postmaster
Canterbury Christ Church University College